The present invention relates to an apparatus for rejecting a jamming wave having a single frequency present in a relatively wide frequency range or in the vicinity thereof in a receiver.
Loran C receivers, for example, sometimes undergo a wave in a Decca navigational system, or so-called "Decca wave" in the vicinity of the frequency band of the Loran C receiver. When a mobile body on which a Loran C receiver is installed enters a service area covered by a Decca wave having a different frequency, it is necessary that the narrow-band rejection filter in the Loran C receiver be re-adjusted to reject the Decca wave. Since the rejection band of the narrow-band rejection filter is quite narrow, it requires increased time and skill to bring the rejection center frequency properly into conformity with the center frequency of the jamming wave. It has also been cumbersome to have to adjust the narrow-band rejection filter each time the receiver enters an area covered by a different jamming wave frequency.
It has been proposed to effect automatic adjustment of the center frequency of a narrow-band rejection filter for rejecting jamming waves into conformity with the frequency of a jamming wave. One such automatic frequency adjustment is disclosed in PCT International Publication No. W081/01930, published on July 9, 1981. When a jamming wave which is an intermittent signal is temporarily turned off, a control output from a control circuit for detecting the difference between the central frequency of the narrow-band rejection filter and the jamming wave frequency is widely deviated from a normal condition. In case the rejection center frequency of the narrow-band rejection filter is controlled by such an erroneous detected output, the jamming wave cannot temporarily be rejected each time the jamming wave is turned off, and the control circuit has to be controlled again to adjust the center frequency of the narrow-band rejection filter so as to coincide with the frequency of the jamming wave. Even fully automatic control needs a period of time for bringing the center frequency into conformity with the jamming wave frequency. Another control mode is known in which the rejection frequency is manually adjusted into substantial conformity with the jamming wave frequency, and then the rejection frequency is automatically brought accurately into conformity with the jamming wave frequency. With such an adjustment process, the operator is required to effect manual control each time the jamming wave is turned off.
The narrow-band rejection filter has generally been comprised of a notch filter. Where the range in which the rejection frequency of the narrow-band rejection filter is variable is relatively wide, frequencies at both ends of the variable-frequency range cannot sufficiently be cut off or attenuated. This difficulty has been found due to the fact that the effective resistance of an inductance coil employed in the notch coil becomes increased as the rejection frequency goes higher, and signals passing through a resistor and a capacitor connected in series with input and output terminals of the notch filter have a large level difference therebetween. A variable resistor shunted across the input and output terminals of the notch filter on the side of the capacitor can be adjusted each time the rejection frequency is altered, thus reaching a sufficient degree of attenuation in any rejection frequency. However, such adjustment of the variable resistor is troublesome.